I would take a wild guess that Animal Crossing like his Wii version has some special built in stuff that allows to make screenshots from the game and then those screenshots are saved with the use of some special(?) codec so no "plain usual PNGs" can be uploaded

However yeah, with SuperCard DSTwo I can do screenshots from both GBA and NDS games without much wuss, I believe there will be possibility to do the same with 3DS games when (if?) proper flashcard will be made (no spoiling~) then whole idea of this site will be kind of killed because who would be crazy enough to not use plain old imageshack where he/she can post screenshots from any game VS special-special site with only one game possible...
Gee sometimes Nintendo scares me, I have a lot of respect to them but sometimes they seriously go insane...